1. Field of the Invention
The present invention relates to a camera system having an image blur prevention function for preventing an image blur caused by camera shake or the like.
2. Description of the Related Art
Optical apparatus for preventing an image blur caused by camera shake or the like are known.
U.S. Pat. Nos. 2,959,088 and 2,829,557, for example, disclose a device for preventing image blur by utilizing the inertia of a movably disposed correcting optical system.
FIG. 1 shows the configuration of the aforementioned type of image blur preventing device. In FIG. 1, in addition to main lenses 12 and 13 fixed to a lens barrel (hereinafter referred to simply as a barrel) 4 for forming an image on a focal plane (not shown), lenses 1 and 2 are provided to form a correcting optical system for correcting image blur. The focal length of the correcting optical system is set such that it satisfies the following relation EQU f1=f2
where f1 is the focal length of the lens 1 fixed to the barrel 4 and has a negative refractive power, and f2 is the focal length of the lens 2 supported on a movable supporting member 3 and has a positive refractive power.
The movable supporting member 3 is supported on the barrel 4 by a gimbal 5 so that it can pivot freely about two axes at a position separated from the image principal point of the lens 2 by the focal length f2 (=-f1).
FIG. 2 shows the structure of the gimbal 5. The movable supporting member 3 for holding the lens 2 is supported by a vertical supporting member 5y which can pivot freely about the y axis, the vertical supporting member 5y is supported by a horizontal supporting member 5x which can pivot freely about the x axis perpendicular to the y axis, and the horizontal supporting member 5x is supported by the barrel 4. In this way, the correcting optical system can pivot freely about two axes.
In the structure shown in FIG. 1, a counterweight 10, which balances the lens 2 relative to the gimbal 5, is mounted on the movable supporting member 3 on the side of the gimbal 5 which is remote from the lens 2.
An inertia pendulum type stabilizer optical system is thus arranged. This stabilizer optical system, which is shown in FIG. 1, prevents an image blur in a manner described below.
Assuming that the structure shown in FIG. 1 comprises a telescope, an optical image of an object is formed on a focal plane 14 within the barrel 4 directed at the object. In the case of a hand-held telescope having a high magnification, the barrel 4 is vibrated at a frequency ranging from 0.1 to 10 Hz due to a phenomenon known as "telescope shake" and this generates an image blur.
However, according to the optical mechanism which is shown in FIG. 1, when such vibrations occur, relative displacement occurs between the lenses 2 and 1 due to inertia of the movable supporting member 3, and image blur is thereby suppressed.
In FIG. 1, a damping member 9 made of a non-magnetic, electronically conductive material, such as aluminum, is mounted on the movable supporting member 3. The damping member 9 generates a damping force in accordance with the speed at which the barrel 4 is vibrated because of the magnetic effect formed by magnets 6 and 7 fixed to the barrel 4. This damping force prevents collision of the movable supporting member 3 against the inner wall of the barrel 4, which would occur when the barrel 4 is rapidly displaced in order to change, for example, the composition of an object being viewed.
Specifically, as shown in the enlarged view of FIG. 3, an eddy current, generated in the electronically conductive damping member 9 due to the presence of the magnets 6 and 7, produces a damping force in a direction corresponding to the displacement of the movable supporting member 3 from the center of the movement, on which the optical axis of the lens 2 coincides with the optical oxes of the main lenses 12 and 13, i.e., a main optical axis 15.
In FIG. 3, the magnets 6 and 7 are shown only on the upper portion of the barrel 4. However, this is merely for convenience of description, and similar magnets are provided on the lower portion and on the right and left portions of the barrel 4 to achieve biaxial control.
Also, a magnetic member 11, formed integrally with the counterweight 10, is mounted on the movable supporting member 3. A magnetic field formed by both the magnetic member 11 and a magnet 8 fixed to the barrel 4, which is shown in FIG. 1, is utilized for centering the movable supporting member 3 on the center of movement on which the optical axis of the lens 2 coincides with the main optical axis 15. When no image blur is present, it is preferable from the standpoint of the optical characteristics that the centering operation be conducted using the central portion of the lens 2. Therefore, manufacturing errors or direct current components of the eddy current are eliminated, and the optical axis of the lens 2 is made to coincide with the main optical axis 15.
Specifically, the magnetic member 11 and the magnet 8 are disposed such that the same magnetic poles, e.g., the north poles thereof, face each other, and are hence arranged such that they magnetically repel each other, as shown in FIG. 3. At that time, since the center of the magnet 8 coincides with the main optical axis 15, a centering force is generated such that the optical axis of the lens 2 coincides with the main optical axis 15.
Thus, the above-described damping and centering structures do not hinder improvement in the characteristics of the inertia pendulum type image blur preventing device.
In recent years, single-lens reflex cameras have generally been used together with any of various types of interchangeable lenses, ranging from a wide angle lens to a telephoto lens. A photographer makes a selection of these interchangeable lenses in accordance with his or her photographic intentions. Also, video cameras of the type which permit a change of lenses have been researched. When the aforementioned image blur preventing device is incorporated on an interchangeable lens mounted on such cameras, image blur, caused by camera shake or the like, may be eliminated.
However, the above-described image blur preventing device has the following drawbacks. During certain photographic operations, panning the camera, i.e., moving the barrel 4 in a horizontal direction in order to change the horizontal composition of the image being viewed or tilting the camera, i.e., moving the barrel 4 in a vertical direction in order to change the vertical composition of the image being viewed is often conducted in order to track or change an object being viewed. However, the above-described image blur preventing device is capable of preventing only vibrations caused by camera shake or the like. Therefore, when a panning or tilting operation is actually conducted by moving the camera continuously in one direction, the image blur preventing effect may be reduced, or the correcting optical system moved in one direction may remain at that position or collide against an inner wall of the barrel 4, thus producing an image blur.
Therefore, mere incorporation of the aforementioned image blur preventing device on an interchangeable lens raises the foregoing problems.